#app4 .circle{
    width: 15vw;
    height: 15vw;
    margin-top: 15vw;
    margin-left: 15vw;
    background: #000;
    border-radius: 50%;
}
#app4 .circle.active{
    animation: change 1s  infinite alternate linear;
}
@keyframes change {
    0%{
        background: #000;
    }
    100%{
        background: #fff;
    }
}